The present invention relates to a fluid reservoir for mounting on a brake master cylinder, and more particularly to a fluid reservoir of the type which includes an extendable diaphragm seal member coupled within a reservoir casing in a fluid-tight manner to close off brake fluid stored in the casing from the surrounding air.
In such a conventional fluid reservoir as described above, the diaphragm seal member is extended upward or downward in accordance with inflation or deflation of the air in the closed space to maintain the pressure in the closed space substantially at the atmospheric pressure. If the pressure in the closed space becomes positive or negative pressure under maximum extension of the diaphragm seal member, it is well known that unexpected troubles will occur in the vehicle braking system. To solve the problems, a conventional diaphragm seal member is provided thereon with inlet and outlet check valves which act to permit the flow of air to and from the closed space. However, it is difficult to uniform the performance of the fluid reservoir by provision of the inlet and outlet check valves.